Field of the Invention
The present invention relates to oil feeding systems for oil filled cables. The oil pressure in such cables has traditionally been maintained using oil reservoirs such as small cylindrical tanks containing mild steel or stainless steel cells.
The reservoir could either be of the low pressure type (either gravity feed or variable pressure) or it could be of the high pressure type (either with pre-pressurized cells, or with the cells manifolded so that the gas pressure could be varied).
Long submarine crossings will normally require high pressure reservoirs, particularly if the water is deep and it is required that the pressure inside the cable always is higher than that of the surrounding water.
If a submarine cable should become severed (for instance by a dragging ships anchor) so much oil could be lost that the capacity of the reservoirs would not be enough during the subsequent cooling period, and water would be sucked into the cable.
In order to maintain the cable free of water, even after a complete severance, it has become customary to use pumping plants rather than reservoirs for important submarine crossings. The pumping plants are generally provided with fairly large size storage tanks, and with a system for reducing the outflow of oil once the cable has been cooled down. Such systems have been designed to keep the cable free of water for periods as long as 60 days.
Most pumping plants depend on a supply of electrical power to operate. To protect the cable even in case of a failure of the power supply, it is customary either to provide the pumping plant with a diesel engine-generator unit or to use a pump driven by compressed gas taken from bottles, as a back-up for the electrically driven pump.
Pumping plants for oF cables generally use a so-called `canned` motor-pump assembly i.e. (the unit is hermetically sealed, and the oil flows through the rotor of the motor) to avoid any possibility of vacuum leaks. These pumps are expensive and require an elaborate control system for starting and stopping to maintain oil pressure within preset limits. An air driven pump on the other hand will only pump when the oil pressure falls below the pressure for which the gas pressure is set, and it will stop pumping as soon as this pressure is again reached.
From U.S. Pat. No. 4,405,292 (Haskel) there is known a pneumatically controlled rate pump system. Upon receiving a pilot signal, the piston type pump will make one stroke and then wait for the next pilot signal. The pump is provided with a counter which records the number of pump cycles and thus the volume of fluid that has been pumped. This results, however, in a rather uneven fluid flow.